(1) Technical Field
The present invention relates to a display driver LSI for driving a display such as a liquid crystal panel, and more particularly to a circuit arrangement for supplying a uniform current to each of the display drivers.
(2) Background Art
In recent years, Flat Panel Displays (FPDs) have been becoming thinner and lighter and costing less with increased screen size and fineness. Against this backdrop, display driver LSIs for driving a display panel such as an FPD are being improved.
FIG. 7A is a diagram schematically showing the structure of a display panel part of a liquid crystal display, FIG. 7B is a circuit diagram showing the structure of a known display driver, and FIG. 7C is a view showing variations in brightness of the display panel. These drawings show an example of a liquid crystal display panel in which gray scale control is performed in accordance with the magnitude of voltage.
As shown in FIGS. 7A and 7B, in a typical TFT (Thin Film Transistor) active liquid crystal display panel, pixels (sub-pixels) 601 each composed of a transparent TFT 602 and a liquid crystal capacitance 603 connected to the TFT 602 are placed in matrix. Each of the pixels 601 is connected to a corresponding drive voltage supply unit located in a display driver LSI 605 and supplied with a voltage for gray-scale control from the display driver LSI 605. The display driver LSI 605 is obtained by integrating, on a single chip, not only a bias current circuit 606 but also plural drive voltage supply units, such as drive voltage supply units 619, 620 and 621. In the case of a large-screen liquid crystal display, a plurality of display driver LSIs 605 of this kind are placed in the frame of the display panel. A circuit including a bias current circuit (current source) and a drive voltage supply unit is herein referred to as a “display driver”.
In this display panel, the level at which display pixels shield backlight varies by changing the voltage value to be applied to the liquid crystal capacitance 603. This leads to a change in display brightness in proportion to the voltage applied from the display driver.
Next, a description will be given of the structure of the known display driver LSI shown in FIG. 7B.
First, a bias current circuit 606 for supplying a current of a fixed value to a drive voltage supply unit 619 includes a first metal oxide semiconductor field-effect transistor (MOSFET) 608 of a first conductive type, a resistor 607 connected to the first MOSFET 608, a second MOSFET 609 constituting a current mirror in conjunction with the first MOSFET 608, and an input transistor 610 of a second conductive type connected to the second MOSFET 609. The input transistor 610 is for inputting current to a current mirroring part located in the drive voltage supply unit 619 that will be described later.
Next, the drive voltage supply unit 619 includes a current addition type digital/analog (D/A) converter 630 having plural current mirroring devices, and a current/voltage converter 611 connected to the output part of the D/A converter 630.
The D/A converter 630 includes a first mirroring device CM1, a second mirroring device CM2, . . . , and an n-th mirroring device CMn each composed of a MOSFET of a second conductive type (in this case, N-channel type) and constituting a current mirror in conjunction with the input transistor 610, and switches L1, L2, . . . , and Ln connected to the first mirroring device CM1, the second mirroring device CM2, . . . , and the n-th mirroring device CMn, respectively (n: natural number). The current/voltage converter 611 consists of an operational amplifier subjected to negative feedback and a resistor. Each of drive voltage supply units 620 and 621 also has the same structure as the drive voltage supply unit 619, and gate electrodes of the mirroring devices of plural drive voltage supply units are connected together via a common conductor.
Next, a description will be given of current flowing through the known display driver.
The bias current circuit 606 of the known display driver can produce a desired magnitude of reference current by controlling the resistance value of the resistor 607. This reference current is distributed to the second MOSFET 609 and then is fed to the input transistor 610. At this time, a current flows through each of a first mirroring device CM1, a second mirroring device CM2, . . . , and an n-th mirroring device CMn. FIG. 7B simply shows the mirroring devices as if each of them is composed of a single transistor. However, they are actually composed of one, two, four, . . . , and 2n−1 transistors of an equal size, respectively. For example, in the case of a 6-bit (64-gray-level) liquid crystal display, 1+2+4+8+16+32=63 transistors are provided in accordance with the weighting of bits. Therefore, in the case where a current flowing through a switch L1 in on position is assumed as I, currents flowing through the switches L2, L3, . . . , Ln when the switches L2, L3, . . . , Ln are on are 2I, 4I, . . . , 2n−1, respectively. Hence, when the on/off switching of each of the switches L1, L2, . . . , Ln is controlled, it becomes possible to feed 2n different levels of current to the current/voltage converter 611. The current/voltage converter 611 converts the fed current into voltage and supplies the resultant voltage to the pixel 601.
Next, a description will be briefly given of the operation of the known display driver.
In the known display driver, display data are held in the form of digital signals (not shown). The switches L1, L2, . . . , Ln are turned on or off depending on these display data. When all of the display data are displayed in white, all of the switches L1 through Ln are turned on. On the other hand, when all of the display data are displayed in black, all of the switches L1 through Ln are turned off.